The present invention relates to phosphors containing oxides of alkaline-earth and Group IIIB metals, and light sources using these phosphors. In particular, the present invention relates to phosphors containing at least oxides of at least a Group-IIIB metal, magnesium, and at least another alkaline-earth metal.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths as close to the visible light wavelengths as possible, and phosphors that respond to those wavelengths. Recent advances in light-emitting diode (“LED”) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term “near UV” as used herein means UV radiation having wavelengths in the range from about 250 nm to about 400 nm. These LEDs emitting radiation substantially in a portion of the near UV-to-blue range will be hereinafter called “UV/blue LEDs.” As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. As used herein, the term “near UV-to-blue” wavelength range means from about 250 nm to about 480 nm. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.
Some UV/blue LEDs based on combinations of nitrides of indium, aluminum, and gallium have recently been developed. For example, U.S. Pat. No. 5,777,350 disclosed LEDs comprising multiple layers of indium and gallium nitrides and p- and n-type AlGaN, which emit in the wavelength range of about 380 nm to about 420 nm. The active layer of such an LED may be doped with other materials to shift the LED peak emission within this wavelength range. An LED having a peak emission in the blue light wavelengths was combined with a coating of a yellow light-emitting yttrium aluminum garnet phosphor activated with cerium (“YAG:Ce”) to produce white light is disclosed in U.S. Pat. No. 5,998,925. Although a substantial portion of the need for white light devices may be filled by LED-based devices, the ability to combine a UV/blue LED with a phosphor has been limited because YAG:Ce has been the only known yellow light-emitting phosphor that is excitable by radiation in the blue range. However, using a single phosphor in conjunction with an LED emission limits the ability of lighting developers in achieving higher efficiency and the freedom to vary the correlated color temperature and/or the color rendering index (“CRI”) of the light source.
Therefore, there is a continued need to provide more efficient phosphors that are excitable in the near UV-to-blue wavelength range and that emit in the visible wavelength range. It is also desirable to incorporate such phosphors in light sources to provide white light with high efficiency and/or high CRI.